Introduction

The seeds reflect their importance in the maintenance of genetic diversity, through them the propagation of the species is made (^{Amador-Alférez et al., 2013}). The seed of good quality is the cornerstone for the establishment of a good population in any crop. The trend of the seed industry is increasingly specialized, with companies dedicated exclusively to the production of good quality seeds. Therefore, an important research area has been created in the search for seeds with higher physical, sanitary, physiological and genetic quality (^{Jahnke et al., 2016}).

^{Evans and Turnbull (2004)} consider that seeds with good physiological quality are those that have a high percentage of germination and vigor, mainly. In addition, it confers advantages such as the extension of the shelf life, a minimum of waste and production of uniform plants in nurseries, seedbeds and fields. ^{Luna et al. (2012)} mentions that physiological quality depends on biotic and abiotic factors that can easily alter maturation, damage the crop, drying, storage, distribution and its establishment in the field; the seed is susceptible to being damaged at any time during its production, so its management from maturation to sowing requires a high degree of specialization.

According to ^{Romano et al. (2008)}, one of the quality components that shows signs of deterioration is the vigor of the seeds, followed by a decrease in the germination percentage, in the production of normal seedlings and finally death. The factors that characterize the vigor of the seeds are rapid germination, speed of development after germination and integrity and normality of the seedlings that emerge from a vigorous seed. Given the importance of the vigor of the seeds, research is carried out to understand the genetic nature and effects on the establishment of seedlings in the field, as well as the effects on the yield and longevity of seeds in storage (^{Sveinsdottir et al., 2009}).

Various theories have been formulated regarding the biological phenomena that lead to seed deterioration. Stresses the research conducted in the last fifteen years, related to the action of free radicals in the deterioration of mitochondrial membranes, as the first event in the deterioration of seeds. Aging alters the activity of the H+ATPase of tonoplast, peroxidase, protease and DNA polymerase (^{Svenisdottir et al., 2009}).

Polyethylene glycol (PEG) has been recommended as an ideal osmotic substance for seeds, because it does not penetrate cell membranes, is not toxic and keeps its osmolarity constant in small quantities, which allows an acceptable aeration of the medium; To avoid the effect of hypoxia in the seed, which causes the accumulation of toxic metabolites (^{Finch-Savage et al., 2004}), it has been found that the behavior of seedlings of monocot species is different from that of dicotyledons if they develop under stress osmotic with PEG. In general, monocots are more sensitive to stress at the level of the radical system (^{Campos-Álvarez et al., 2002}; ^{Méndez-Natera et al., 2008}).

^{Lara et al. (2014)} found that nitrates stimulate germination by decreasing ABA concentration and nitric oxide production, thus interrupting dormancy, as well as stimulating antioxidant enzymes. ^{Ilbi and Eser (2002)} point out that KNO₃ is involved in protein synthesis and carbohydrate metabolism, and consequently, delay seed aging. In this investigation the effect of osmoaconditioning with potassium nitrate (KNO₃) (3%) and gibberellins (GA3) (0.06%) in two periods: 6 and 12 h, on the seed subjected to accelerated aging in six maize populations was evaluated, including an experimental hybrid.

Results and discussion

There were no significant differences between the control and the treatments in any of the times for the Roque B16 Hybrid, none of the parts in this seed was sensitive to deterioration, since as ^{Pérez de la Cerda et al. (2007)}, the different parts of the seeds do not deteriorate or age simultaneously and those most sensitive to deterioration should be the focus of attention in quality studies. The endosperm of maize is mainly composed of starch, which is not as sensitive to deterioration as the embryo; this is mainly composed of proteins and oil, while the endosperm represents 80% of the dry weight of the maize seed, 20% refers to the embryo.

It is known that proteins, lipids and DNA are more sensitive to damage. The type of endosperm influences the germination and vigor of the seed, corneal endosperm maize resists deterioration better (^{Pérez de la Cerda et al., 2007}). For this B16 genotype, hybrid vigor would explain the resistance to seed deterioration, as indicated by ^{Greaves et al. (2015)}. In the imbibition variable, the 12 h treatment absorbed a greater amount of solution; as the soaking time lengthens the seeds absorb more water (Table 1).

^{Sousa Paiva et al. (2006)} indicated that, during imbibition, the increase in the weight of the Swietenia macrophylla seeds was only significant in the first two phases; that is, that of rapid absorption and then not absorb water for a certain time (until the protrusion of the radicle). In electrical conductivity, the Roque B16 seeds soaked for 6 and 12 h exceeded the control (5.54 µs/cm), the electrical conductivity is based on the principle that as the seeds deteriorate, the differential permeability of cell membranes is altered, so that, when placed in an aqueous solution, the less vigorous seed suffers loss of cytoplasmic compounds such as amino acids, ions and sugars of low molecular weight, when the seeds are rehydrated they must have the ability to reorganize the membranes to avoid the loss of solutes (^{Yu et al., 2015}).

Roque B16 values were similar to those reported by ^{Hilmig and Mendez-Natera (2011)}, both unaged and aged seeds, with electrical conductivity values of 20 to 32 µs cm^-1.

The results for the Elotes Occidentales race (purple pigmentation) are shown in Table 2; at 7 and 30 days after the treatment, no differences were observed between the control (non-osmoaconditioned seed) and the treatments (osmoaconditioned and subject to accelerated aging), at 60 days the control had a germination percentage higher than the treatments, there was a negative effect of the treatments since the germination was too low and lower than the control, at this stage the germination was less than 85% in control and treatments.

The endosperm of this seed is floury, which explains the negative effect of accelerated aging on germination and agrees with the results of ^{Pérez de la Cerda et al. (2007)} who point out that the corneal endosperm confers more resistance to deterioration and with ^{Duran-Hernández et al. (2011)} who indicate that the seed with greater vigor resists accelerated aging better. Germination and vigor are quality indicators, they are attributes that allow us to assess the establishment of the plant in the field.

The size, shape and type of endosperm, vitreous or mealy are important characteristics in a seed. 35 maize populations evaluated by ^{Guillén-de la Cruz et al., (2018)} showed a great variation in seed germination ranging from 28-100%. The populations with vitreous endosperm tended to have greater vigor, germination and normal seedlings than those of floury endosperm. Pigmented maize is seeds with a flour-like endosperm (^{Gómez et al., 2017}). ^{Gutiérrez-Hernández et al. (2011)} report that the Oaxaca maize variety has a good behavior, since it produces normal seedlings with seeds subjected to humid heat, which contrasts with the results obtained here.

Maize seed contains more than 80% unsaturated fatty acids, as part of total lipids, which makes it more susceptible to deterioration (^{McCann et al., 2007}). Table 2 shows the results of the Elotes Occidentales (red pigmentation) race, which shows that after seven days, the osmoaconditioning at 12 h caused a significant decrease in germination; at 60 days the germination percentage of the two treatments decreased (6 and 12 h), which were lower than the control; in this variety, germination in all cases was less than 85%.

The degradation of oligosaccharides during imbibition could explain the low germination, in addition to stress due to the accelerated aging test, as indicated by ^{Gurusinghe and Bradford (2001)}. In nature, the seeds must be able to enter cycles of hydration and dehydration and be able to germinate when conditions are conducive (^{Footitt et al., 2015}), but the domestication process seems to have suppressed or diminished this property.

There were significant differences between the treatments and the control in the variables imbibition, electrical conductivity and ashes in the population of Elotes Occidentales blue pigmentation (Table 3).

Table 4 shows the comparison of means of the Roque Amarillo population, treatments at 6 and 12 h had a reduction in germination; they were inferior to the control. This behavior was similar at seven and 60 days. In the control and in the treatment at 6 h, germination was at least 85%. At 30 days no differences were observed between the treatments and the control. Apparently, the corneal characteristic of the endosperm influenced to obtain a good level of germination in this population, in comparison with the floury endosperm populations that showed low germination (^{Pérez de la Cerda et al., 2007}).

^{Gutiérrez-Hernández et al., (2011)} mention that there are different degrees of tolerance to accelerated aging; seed exposure to dry heat causes fewer normal seedlings, less accumulation of dry matter and higher proportion of dead seeds. Table 4 shows the comparison of means of the White Well, the 12-hour treatment had a negative effect on germination and was lower than the control in the three storage times, the six-hour treatment was statistically the same as the control seven and 30 days, but at 60 days, this treatment showed a negative effect and was statistically inferior to the control.

This population is floury endosperm, which affects germination over time, reflected in a very low germination. ^{Villaroel and Méndez-Natera (2007)} point out that the growth conditions affect the resistance of the maize seed to storage conditions, those in cold areas better resist refrigeration, this and the type of endosperm could be part of why these researchers found a favorable effect of accelerated aging on germination, different from the results obtained in this research.

In electrical conductivity, the treatments exceeded the control in Roque Amarillo, the electrical conductivity estimates the integrity of the cell membrane, the losses of cytoplasmic solutes with electrolytic properties are indicative of the rapid deterioration of the seeds (^{Hilmig and Méndez-Natera, 2011}). The treatments of 6 and 12 h did not show differences between them (Table 5), the potassium nitrate did not influence the fluidity to the cell membranes. ^{Ashley et al. (2006)} indicate that potassium plays an important role in basic cellular functions; as well as in the regulation of osmotic pressure and membrane potential.

The pH of the seeds was increased at 6 and 12 o’clock, beating the control (6.15). ^{Hilmig and Méndez-Natera (2011)} reported a very narrow pH range, between 4.9 and 5.6 in maize seeds; the seeds that recorded a pH greater than 5.8 were killed; however, the pH obtained in this investigation were higher than those reported by this author, and it differs in that the seeds of this case were not killed.

As a percentage of ashes, the 6 and 12 hour treatments presented values higher than the control by 18%. ^{Sánchez (2004)} determined ash contents from 1.4 to 1.54% in five yellow maize genotypes, in the data presented here it is clearly observed that the control showed lower ash content, while in the treatments the percentage of ash increases, the values found here are in accordance with the values reported by the aforementioned author, the application of potassium nitrate improved the ash content in the treated seeds (Table 6).

The ^{FAO (1985)} points out that the mineral that is most abundant in the seed is phosphorus in the form of potassium and magnesium phytates, being entirely in the embryo, with values of approximately 0.9% in common maize and about 0.92% in opaque maize-2.

In six hours imbibition maize showed superior absorption Pozolero 12 h and the control, in a first phase, the seed absorbs water proportionally to the soaking time, but there comes a point where the absorption decreases, at 12 h soaking, the seeds decrease the absorbed water, which was observed by ^{Méndez-Natera et al. (2008)} for the maize, caraota (bean) and quinchoncho (pea) crops with rate increases until 8 and 10 h after imbibition, respectively (Table 7).

In electrical conductivity, the control (8.74 µs cm^-1) showed lower ion output compared to treatments. Nitrate is indirectly involved in protein synthesis and carbohydrate metabolism and in delaying seed aging. Its effect on tomato seeds (S. lycopersicum) increased the total protein content and nitrate reductase activity, although it did not affect the germination percentage, uniformity and synchrony. The treatment effect could be observed, positively, in low quality seeds (^{Lara et al., 2014}).

Figure 1 shows the means adjusted for minimum squares. It is observed that the Amarillo and hybrid varieties were statistically equal with a higher percentage of germination during storage with little effect of the latter. The Morado and Pozolero varieties were greatly affected by the three-month storage. This is because its endosperm is starchy, the Rojo variety was also affected but to a lesser extent than the previous two; this effect of decrease in germination by storage time was notorious until the third month.

Figure 1 Illustration of the interaction variety x storage time in the germination variable. Germ= germination percentage; var: variety; AMA= yellow; HIB9, hybrid; MORA= Elotes Occidental Morado; PALO= palomero maize; POZO= Pozolero white maize; ROJO= Elotes Occidentales Rojo; trat= treatment 1 (0 h), 2 (6 h) and 3 (12 h). 

According to ^{Finch-Savage et al. (2004)} the accumulation of harmful metabolites, during the soaking test under anaerobic conditions, the percentage of germination decreases, especially in sensitive cultivars. ^{Ventura et al. (2012)} point out that the beneficial effect of osmoaconditioning can be subsequently lost due to oxidative stress.

In the Palomero the variables imbibition and electrical conductivity in the two treatments were not significantly different (p≥ 0.05) but exceeded the control; maize seed usually has an acidic pH, which ranges between 4-5. The 6-hour treatment reached values of 6.16, some authors point out that the pH variance of the maize is a condition for the quality of the seed, since very small alterations usually activate the proteins and enzymes of the internal structure mainly embryo.

The analysis of variance of the Palomero Toluqueño population for the standard germination variable indicated that there were no significant differences between treatments. The percentage of ashes, the two treatments are not significantly different (p≥ 0.05), but are higher than the control, the application of potassium nitrate had a significant effect (Table 8).

Figure 2 shows that the treatment time at 12 h most strongly affected the Palomero, Amarillo and hybrid population were the least affected with the treatments, had more germination than the other populations. Morado and Palomero behaved similarly regarding the treatment. Palomero and Rojo behaved similarly.

Figure 2 Illustration of the interaction variety x treatment of osmoaconditioning in the germination variable. Germ= germination percentage; var: variety; AMA= yellow; HIB9, hybrid; MORA= Elotes Occidentales Morado; PALO= palomero maize; POZO= Pozolero white maize; ROJO= Elotes Occidentales Rojo; trat= treatment 1 (0 h), 2 (6 h) and 3 (12 h). 

There is a clear effect of the treatments on the germination of the maize seed. The treatment at 12 affects in a more pronounced way the germinative capacity of the seed, so other times and other substances could be evaluated to discover any substance that could have a positive effect on the germination and vigor of the seed, which also help withstand aging, as described by other researchers (^{Finch-Savage et al., 2004}; ^{Villaroel and Méndez-Natera (2007)}, although it is difficult to determine the correct imbibition time to then dry the seed without suffering damage (^{Ventura et al., 2012}).

Conclusions

Osmoconditioning did not improve seed resistance to accelerated aging damage. There was a negative effect of the treatments on the germination of the seed, although the one of hybrid origin resisted the treatments, followed by the one of corneal endosperm (Roque Amarillo), the Palomero, although it is of hard endosperm it did not show the resistance of the mentioned varieties, but neither is the susceptibility of soft endosperm, Elotes Occidentales and Pozolero. Osmoconditioning at 12 h has a more adverse effect than treatment at 6 h. The results support the proposal that accelerated aging gives a good idea of the physiological quality of the seed.